Mixing circuit



R. F. POST MIXING CIRCUIT July 8, 1952 Filed Jan. 30, 1946 glvwanto'o RICHARD E POST y 5112mm;

Patented July 8, 1952 NITED STATES This invention relates to a novel signal mixing circuit which is adapted to produce a beat frequency difference signal output from a pair of input signals of distinct frequencies.

Previously, in mixing circuits producing several signal components, a sharply tuned rejection filter system has ordinarily been necessary in order to isolate a single desired beat frequency com-- ponent from the remaining signal components present in the circuit output. Consequently, in cases where the beat frequency components'are varied over an extended frequency range the rejection filtering system becomes increasingly more complex. Additionally, the use of arejection filtering circuit ordinarily requires that the elements comprising the rejection filter be retuned for each new beat frequency component.

It is an object of this invention to provide a signal mixing circuit which is rendered free of the foregoing disadvantages.

It is a further object of this invention to provide a signal mixing circuit which will eflecamended April 30, 1928; .370 0. G. 757) 1 tively isolate the input circuit for a first input signal from the input circuit for a second input signal.

Other objects and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings in which the single figure is a detailed circuit diagram of a preferred embodiment of the invention.

In accordance with the present invention, a mixing circuit is provided which comprises a pair of vacuum tubes, each of which is adapted to receive a separate input signal of distinct frequency or a band of frequencies; The tube components are so cross-connected and arranged relative to a common impedance element, across which the circuit output terminals are taken, that the original, or input, signal frequencies are balanced out across the output terminals thereby leaving only the beat frequency components available at this point. As hereinafter described in detail the beat frequency difference signal is the only signal component which is ofinterest. Consequently,

the beat frequency sum signal is eliminated from the circuit output terminals by means of a suitable by-pass capacitor.

The invention may be better understood by referring more specifically to the single figure in the accompanying drawing. In the drawing the tube components comprising the preferred embodiment of the invention are indicated at [2 and I5, with the plate of each tube cross-connected to the cathode of the other via capacitors l9 and 2c. Each of the tubes is plate loaded by resistances H and I4, respectively, and cathode loaded by variable resistances l3 and i6, respectively. The common impedance element'across' which the output terminals 25 are taken is represented as a suitable resistance element ll, which connects the junction points of I resistances l3 and H5 to ground.

In the circuit of the drawing, a first signal of one frequency or band of frequencies is impressed at terminals Ill to the grid of the vacuum tube l2 and a second signal of a second frequency or band of frequencies is-impressedat terminals 2! to the grid of tube I5. The instantaneous polarityof the signal introduced at the grid of tube i2 is represented, for purposesof illustra tion, as arrowhead A. This signal is amplified at the plate and at "the same time displaced 180" degrees in phase by normal amplification action at the plate, as shown by arrowhead B. However the signal represented by arrowhead C appear-' ing at the cathode of this tube is'in phasewith the original'signal A, since the cathode resistor l3 brings about a normal cathode follower action.

' A portion of the cathode signal C will also appear at the cathode :of the tube it, due to the couplingv action of the common cathode resist= ance 11. head D. The plate signal B of the tube I2 is also coupled to the cathode of the tube I5 by the coupling means l9, this signal being represented as arrowhead E. It will be seen that plate signal E is opposite in phase to cathode signal D and the two will tend to cancel. 7 Thus, by making the plate resistors H and It, or the cathode resistors 13 and 16, or even the coupling capacitors l9 and 20 variable the signal component applied at terminals ill and appearing at the cathode of tubes l2 and is can be set so as to cancel out across the common cathode resistance 11. For purposes of illustration only the cathode resistances I3 and it are made variable. In cancelling out the signal component applied at terminals l0 and appearing across output terminals 25 the-signal phase represented by arrowhead E must pre-' duction of beat-frequency signals is facilitated by This signal is represented'as' arrow-' 3 the non-linearity of the tube l2, produced by the the phase opposition of signals B and C. It can be readily understood that the action at the put signals is desired, this filteringcan heattain'ed; .15-

by connecting a by-pass condenser 18 across the resistor IT. This condenser is especially useful in filtering out the summationdoeat frequency"; signal when only the difference beati'frequency is: desired. If this summation beat frequency is 20 high enough; the. signal may be additionally: filteredby the interelectrode tube; capacitance, whichwill have .low reactances as. comparedzto. the. plate resistors .l l and. ll. and the. cathode resistors :I 3. and 16.;-

In: a specific example of..this..circuit,- a type GSNI tube, having twin vtriodes a in ..a single en-x velope, was used as the tubes-Hand; I5. Cathode resistors l3 and I5 were.variableresistanceswith a maximumyalue-of15,000 ohms;-plate resistors H and -14 were bothyalued at:18,000 ohms; the outputmeansl'l was a 22,000 ohm. resistoryand. thetcouplingmeans I 9 and. 20. were: 0.01 mid. and 150.. mmfd.-. capacitors, respectively. One. input signal had a frequency, of: 120.kilocyoles?and. the. 5 other.-. ,wasvariable between" 13.0and. 180 kilo, cycles Whileinnly .a .certain :preferred; embodiment. of. thisinventionchas; been .described,. his realized that many modificationsand variations of..this invention-maybe made and no limitationsupon this invention are; intended other than may be imposed by the scope ofqtheappended claims.v

The. invention described herein maybe manufactured-and. used by or for..the.Government .of the..-United..States "of;- America :for. governmental purposes without. the payment of any royalties thereon or therefor.

Whatis. claimedis: I

1. Airequency mixing circuit comprisinga pair of vacuum-tubes eachhavingatleast an anode,- a cathode and a grid, means for -.introducinga first :signal .tojoemixedto.;the grid of one .of said tubes;- and. a second-signal :to. be mixed: to: .the grid of:..-the tother-.tube, coupling means cross: connecting the anode of each tube tothe cathode of the. other-.tube, resistive means connecting the cathode of each. of said'tubestosa common .terminal, resistivemeans connected. from the plate of each. of.- saidtubes to a commonplate voltage, supply, and output. meansconnected to. the com mon: terminal-ofsaid cathode resistors.

2. A frequency-mixing circuitcomprising a pair. of vacuum tubes each having at. leastan anode, .a grid. and; a cathode, impedance means-.cross-con-z necting the anodeof: each tubesto theicathode of- 4 the other, a separate input terminal coupled to the grid of each of said tubes, and a single impedance coupling mean connected in common with both of said tubes and across which a pair of output terminals are connected.

3. A frequency mixing circuit comprising a pair of vacuum tubes each having at least an anode, a :grid. and. a cathode, impedance means crossconnecting the anode of each tube to the oathodaof the other, a separate input terminal coupled to the grid of each of said tubes, and a single impedance coupling means connected in commonwiththe cathode circuits of said tubes and across which the output from the circuit is takenn.

4. A- frequency mixing circuit comprising, a pair "of' vacuum tubes each having at least an anode, a grid. anda cathode, capacitance means cross connecting theanode of each of said tubes to the cathode of the other, a separate input means coupled to the grid of. each of said tubes and to .whichthesignals to. be mixed are ap plied, and resistance means connecting bothof said tubes across-a source of operating potential,

said. resistance means including a resistance element connected in common to both of said. tubes.

and across which the output from the circuit is taken.

5. A frequency mixing circuit comprising, a pair of vacuum tubes each having at least an anode. a grid and a cathode, capacitance means cross connectingthe anode of each of said tube .to the cathode of the other, a separateinput means cou-..

pled to the grid of each of said tubes and to.

which the signals to be mixed are applied, a sepa- A rateresistance element connecting the anode of each .of, said tubes to a source of operating potential, a separate variable resistance connecting the cathode of each tube to a common junc-.

tion point, and a parallel combination ofresiste ance and capacitance connecting said junction point. to ground andacros which the output from the circuit is taken;

6. A frequency. mixing circuit comprising a pair of vacuum tubes each having at least anode, grid, and cathode electrodes, coupling means connecting the anode .of each tube to the cathode of the.

other, a separate input. connection to one electrode of each of said tubes for receiving the signals to be mixed, and. a common output load means coupled to both of said tubes'across which the output from the circuit is derived.

RICHARD F. POST.

REFERENCES CITED The following references are of record in the file of this patent:'

UNITED STATES PATENTS Number Name Date 2,519,030 Dome Aug. 15, 1950.

FOREIGN PATENTS Number Country Date 449,470 GreatBritain June 24, 1936 

